1. Field of the Invention
The present invention relates to an apparatus for controlling the rotational speed of an internal combustion engine at a predetermined target rotational speed according to a feedback control process by controlling the ignition timing of the internal combustion engine.
2. Description of the Related Art
It is the general practice to control the actual rotational speed of an internal combustion engine at a predetermined target rotational speed according to a feedback control process, e.g., while the internal combustion engine is operating in an idling mode, by controlling the amount of intake air introduced into the internal combustion engine. The actual rotational speed of the internal combustion engine can also be adjusted by controlling the ignition timing of the internal combustion engine. Specifically, if the amount of intake air is constant, then as the ignition timing is more retarded, the actual rotational speed of the internal combustion engine is reduced, and as the ignition timing is more advanced, the actual rotational speed of the internal combustion engine is increased. Accordingly, the ignition timing may be controlled to converge the actual rotational speed of the internal combustion engine to a desired target rotational speed according to a feedback control process.
The applicant of the present application has proposed the following technique in Japanese laid-open patent publication No. 10-299631, Japanese laid-open patent Publication No. 11-210608, and U.S. patent application No. 09/063732, for example.
According to the disclosed technique, in order to quickly increase the temperature of and activate a catalytic converter disposed in the exhaust system of an internal combustion engine while the internal combustion engine is idling after it has started to operate, the amount of intake air introduced into the internal combustion engine is increased to a level greater than while the internal combustion engine is idling, for thereby increasing the amount of heat energy of exhaust gases that are emitted from the internal combustion engine when the air-fuel mixture is combusted in a combustion chamber of the internal combustion engine. A command value for the ignition timing of the internal combustion engine is generated according to a feedback control process using a PI (proportional plus integral) control process in order to converge the actual rotational speed of the internal combustion engine, which tends to increase due to the increased amount of intake air, to an optimum idling rotational speed. Based on the generated command value, the ignition timing of the internal combustion engine is controlled to control the actual rotational speed of the internal combustion engine at the desired idling rotational speed.
Various studies made by the inventors of the present application have revealed that when the ignition timing of the internal combustion engine is controlled to control the actual rotational speed of the internal combustion engine at the target idling rotational speed according to a feedback control process (hereinafter referred to as an "ignition timing control rotational speed F/B control process"), the ignition timing has a relatively large effect on the combustion status of the air-fuel mixture in the internal combustion engine and the behavioral characteristics of the internal combustion engine. Therefore, it is highly necessary to accurately control the ignition timing in a manner to match operating conditions of the internal combustion engine, depending on the difference (hereinafter referred to as a "rotational speed difference") between the actual rotational speed of the internal combustion engine and target idling rotational speed.
For example, the rate of a change of the actual rotational speed of the internal combustion engine to a change of the ignition timing tends to be greater as the ignition timing is more retarded. Therefore, when the ignition timing of the internal combustion engine is relatively largely retarded, if a change of the ignition timing (a corrective quantity of the ignition timing) dependent on the rotational speed difference according to the ignition timing control rotational speed F/B control process is excessively large, meaning that the feedback gain of the ignition timing control rotational speed F/B control process is excessively large, then the actual rotational speed excessively changes to the target rotational speed, meaning that the rate of reduction of the rotational speed difference is excessively large, and the actual rotational speed is liable to be unstable (the actual rotational speed is liable to fluctuate in an oscillatory fashion with respect to the target rotational speed).
Conversely, when the ignition timing of the internal combustion engine is controlled to be advanced, if a change of the ignition timing dependent on the rotational speed difference is excessively small, i.e., the feedback gain is excessively small, then the rate of reduction of the rotational speed difference becomes much small so that the actual rotational speed cannot be converged quickly to the target rotational speed.
For example, if the ignition timing is advanced, if the ignition timing control rotational speed F/B control process is performed while the internal combustion engine is idling after it has started to operate, then in an initial stage of operation in which the ignition timing control rotational speed F/B control process is carried out, i.e., immediately after the internal combustion engine has started to operate, the combustion status of the internal combustion engine is apt to be unstable. Therefore, abruptly changing the ignition timing in order to converge the actual rotational speed to the target rotational speed causes the combustion and emission statuses of the internal combustion engine to be impaired and makes the operating conditions of the internal combustion engine unstable.
In view of the foregoing, if the ignition timing control rotational speed F/B control process is carried out according to a PI control process as proposed in the above technique by the applicant, then it is preferable to variably establish a proportional gain and an integral gain (coefficients relative to proportional and integral terms of the PI control process) which defines the feedback gain of the PI control process, depending on required conditions such as the operating conditions of the internal combustion engine.
According to the PI control process, however, when a command value for the ignition timing depending on the rotational speed difference is determined by the processing of the PI control process, and the ignition timing is controlled by the command value, it is generally difficult to predict behaviors with which the actual rotational speed of the internal combustion engine is converged to the target rotational speed. Stated otherwise, if the actual rotational speed is to be converged to the target rotational speed in a desired behavioral manner, it is difficult to predict what values the proportional and integral gains should have to achieve the desired behavioral manner.
Therefore, the values of the proportional and integral gains, which are to be established depending on the operating conditions of the internal combustion engine, have to be determined in a trial-and-error fashion via various experiments or the like. The determination of the values of the proportional and integral gains thus needs a large expenditure of labor, and it is difficult to determine the values of the proportional and integral gains finely and accurately depending on various operating conditions of the internal combustion engine.